Multi section laser diodes are well known in the art and can be switched between different wavelengths. Typically the diode is calibrated at manufacture to determine the correct control currents that should be applied to the laser so as to effect the desired output frequencies from the laser. Degradation in a tunable laser is normally due to dislocations or changes in the material structure of the laser which leads to less injected carriers entering the cavity of the laser and instead passing through the regions surrounding the cavity. These injected carriers that no longer enter the cavity have the effect of reducing the efficiency of the laser as these carriers can no longer contribute to changing the output of the laser.
One of the first known multi-section laser diodes is a three-section tuneable distributed Bragg reflector (DBR) laser. Other types of multi-section diode lasers are the sampled grating DBR (SG-DBR), the superstructure sampled DBR (SSG-DBR) and the grating assisted coupler with rear sampled or superstructure grating reflector (GCSR). A review of such lasers is given in Jens Buus, Markus Christian Amann, “Tuneable Laser Diodes” Artect House, 1998 and “Widely Tuneable Semiconductor Lasers” ECOC'00. Beck Mason.
FIG. 1 is a schematic drawing of a typical SG-DBR 10. The laser comprises of back and front reflector sections 2 and 8 with an intervening gain or active section 6 and phase section 4. An anti-reflection coating 9 is usually provided on the front and rear facets of the chip to avoid facet modes. The back and the front reflector take the form of sampled Bragg gratings 3 and 5. The pitch of the gratings of the back and front reflector vary slightly to provide a vernier tuning effect through varying the current supplied to these sections. The optical path length of the cavity can also be tuned with the phase section, for example by refractive index changes induced by varying the carrier density in this section. A more detailed description of the SG-DBR and other tuneable multi-section diode lasers can be found elsewhere Jens Buus, Markus Christian Amann, “Tuneable Laser Diodes” Artect House, 1998.
As detailed above such tunable semiconductor lasers contain sections where current is injected to control the output frequency, mode purity and power characteristics of the device. Various applications in telecommunications/sensor fields require that the laser can operate at points in a predetermined frequency/wavelength grid; moreover many applications require the power output of the device to be within a defined tolerance for each operating point, and in general, the operating points must be distanced from mode jumps and have high side-mode suppression. In order to provide lasers for such applications, each individual device must be characterised to the desired specification, so there is a corresponding need for a system or algorithm to map the output of the laser over a range of operating currents. For characterisation of lasers in production environments, such a system must also be fast, reliable and automated.
When the calibration is completed a set of operating points are obtained where each operating point corresponds to a required frequency of operating of the laser. The problem exists that as the laser degrades the characteristics of the laser will change relative to this set of operating points and a method to update the table to take these changes into account is required. This invention describes a method where this can be achieved.